The trap is sprung. That’s the view taken by James Forsyth in the Mail on Sunday today. According to his sources, Cameron’s acquiescence to Gordon Brown’s last-minute devolutionary bribe – which I strongly criticised on this site – was a “short-term” defeat intended to “put Labour in a vice”.

Now I don’t necessarily buy the narrative being put about by Cameron’s allies that this is some kind of “strategic victory”. Paul Goodman has made the case that the Prime Minister appears incapable of strategic thinking, instead relying on a succession of – often very impressive – tactical gambits. The fact that Cameron has opened the biggest and most potent constitutional Pandora’s Box going – described by one Tory grandee as “another Europe” in terms of its potential impact on the Conservatives – in order to ruin Ed Miliband’s party conference is yet more evidence of this.

Will Hutton of the Observer is probably right to complain that the Prime Minister and the Chancellor have struck out into unknown constitutional waters in order to “organise the state around the interests of one party. But seriously, who did that first?

Despite the fact that Labour writers like Dan Hodges believe that ending Scottish votes on English legislation will kill the Labour Party, Labour’s own constitutional reforms have made opposing it so untenable that several Labour MPs are already breaking ranks in support of the change. Culpable as Cameron is for continuing a pattern of short-term, party-focused and unstable constitutional reform in this country, he is only setting off the fuse on a powder keg that the Labour Party has carefully positioned beneath its own foundations.

The West Lothian Question, without wanting to diminish the glory rightfully and belatedly accruing to the marvellous Tam Dalyell (who is the only Scottish Labour MP to have been consistently correct on the impact of devolution and was long mocked for being so), has been a feature of the constitutional debate since the end of the Nineteenth Century. Dominic Lawson in today’s Sunday Timesexplains the problems Gladstone faced when he first proposed having Irish MPs sit on ‘Imperial’ matters but not Home Rule issues. It has never been satisfactorily resolved.

When Labour became very enthusiastic about devolution, it decided to proceed without answering this question. Its solution was to “stop asking it”.

Whilst obviously neither strategically nor morally watertight, this wasn’t a ridiculous suggestion in 1999 because the devolved chambers wielded relatively little power. The great majority of the business of the nation was still conducted in the national parliament at Westminster.

However, this is where other failures in Labour’s devolution settlement come into play. Reluctant then as now to risk competition on things like taxation, Labour left the devolved assemblies with power but no responsibility. It did not take local politicians long to cotton on to a potent strategy: promise local people the world, blame remote and stingy Westminster for being unable to provide it, and find a solution for this “problem” in the expansion of their own power and prestige.

The devolved parties thus came at times to resemble competing tribes of political Santas, and the whole process took on a striking resemblance to the Trotskyist ‘transitional demand’, and nationalist and unionist parties united for fifteen years behind a single rallying cry: “More powers!”

Not only did this pose political problems, but it created a process of cultural drift which has seriously eroded the sense of common Britishness which is the Union’s essential cement. As Forsyth and Fraser Nelson write in the Spectator:

“But rather than strengthen the Union, devolution weakened it by creating separate national conversations. National newspapers began to produce Scottish editions — they were a commercial success, but meant the people of Britain knew less and less about each other.”

The critical way that the refusal to answer the West Lothian Question exacerbated this was that the drive for “more powers” – and the attendant hollowing out of the Union in Scotland and Wales – threatened no immediate diminution of the power and prestige of Labour’s vital cadres of Scottish and Welsh MPs.

Not only did this mean none of them had much motivation to step into the “small ponds” of Holyrood and Cardiff Bay (with disastrous consequences in Scotland), but nor did they have any motivation to defend the legitimacy of Westminster government to their constituents or provide a counterweight to the “more powers” drive. As a result, their constituents drifted away from men and women who had increasingly smaller roles in the politics of their day to day lives, and the Union withered.

Indeed, the only time you’d hear most of these MPs speaking out for Westminster and the legitimacy of British government is when someone proposed to stop them voting on issues they had wilfully agreed were none of their business in their own constituencies. But in the long run, maintaining that they had a proper role in deciding on the education and healthcare systems of England, but not the education and healthcare their own constituents received, was always unsustainable.

When I first advanced this line of argument, Twitter dismissed it because it contradicts the common narrative that the national party system was undermined by the evil Tories being evil. But how did that impression come about? British public attitudes surveys reveal that there are no substantial differences between the English, Scottish and Welsh on the vast majority of political issues. During the height of the apparent Conservative dark age cities like Cardiff returned three Tory MPs, and in 1992 Scotland thanked the Conservatives for 11 years of Margaret Thatcher by giving them one (nearly two) extra Westminster seats.

It was Labour politicians, more consumed by partisan hatreds than concern for the nation, who decided to cast Conservatism as an alien doctrine, and by extension England as an alien land. In order to buttress their Celtic fiefdoms with devolved institutions they made the case that they were needed to protect Scotland and Wales from ‘foreign’ right-wing forces.

This parochialism was evident during the referendum when many Scottish Labour figures couldn’t bring themselves to campaign alongside the Conservatives for the very existence of Britain. ‘United with Labour’, indeed.

Viewed in that light, the SNP’s breaking into Labour’s working class base looks in retrospect to have been almost inevitable the moment Scottish Labour appeared to fail in its self-appointed task of guarding those people from foreign Tories. Say what you like the Conservatives, but even if they don’t back our party any more our old voters, from the rural North East of Scotland to the owners of bought council houses, remain the strongest unionist constituencies in Scotland. Our case for the Union was always less transactional than Labour’s, and our former supporters loyalty too it has proved reciprocally steadfast.

For decades now, the Labour Party has hollowed out the Union both culturally and institutionally whilst trying to govern it as if they had not. Like metaphorical frogs, they either failed to notice or wilfully ignored the waters heating up all around them until they boiled over on Friday morning. They can’t say they weren’t warned.